This invention relates to a detector for use with an x-ray tomography apparatus such as an x-ray CT apparatus.
As shown in FIG. 4 as an example, an x-ray CT apparatus comprises an x-ray tube 31, a collimator 34 and a radiation detector 35 such that a fan-shaped x-ray beam 32 emitted from the x-ray tube 31 passes through a target body 33 to be examined and, after passing through the collimator 34, is detected by the radiation detector 35 having many detector elements arranged in an array. Explained more in detail, the radiation detector 35 may be a solid radiation detector formed as a one-dimensional array of combinations of a scintillator for converting radiation into light and a photoelectric converter element for converting light into an electrical signal. An array of 8 to 30 such combinations of a scintillator and a photoelectric converter forms a block, and the radiation detector 35 is formed with several of such detector blocks 36 disposed continuously as a part of a polygon on the circumference of a circle.
As shown in FIG. 5, each block of the radiation detector 35 has many radiation detector elements arranged in a channel direction (indicated by double-headed arrow), mutually separated by screening plates 45, each radiation detector element comprising a rectangularly elongated scintillator element 41 and a photoelectric element 43 such as a photodiode having the same width as the scintillator element 41, pasted to the bottom surface of the scintillator element 41 and disposed on a supporting member 44.
When the fan-shaped x-ray beam 32 emitted from the x-ray tube 31 reaches this radiation detector, the x-rays are converted into light by the scintillator elements 41, this light is then converted into electrical signals by the photoelectric conversion elements 43 such that data can be obtained both in the channel direction and in the slice direction (indicated by another double-headed arrow in FIG. 5). An image is formed on the basis of these data.
With modern x-ray CT devices, however, the sphere of the x-ray tube is large, and this makes the displacement of its focal point also large corresponding to temperature variations. If the focal position moves in the channel direction, its effects are negligible according to the prior art technology described above because there are many radiation detector elements arranged in this direction. If the movement is in the slice direction, say, from Focal Position A to Focal Position B shown in FIG. 5, however, the light-receiving position by the scintillator element changes from its center part to an edge part, and this affects the sensitivity of the detection signal.
As shown in FIG. 6, the sensitivity curve of a detection element is flat near the center but it is not flat but decreases as one moves in the slice direction and reaches an edge position. Thus, if the light-receiving position of each scintillator element moves from the center to an edge position, the sensitivity of the detection signals is adversely affected and the outputted signal is not accurate. This was a cause of the generation of virtual and false images.
Especially when data of a thick slice are to be obtained, the difference in sensitivity between the center and edge parts of the detection elements in the slice direction is large, and virtual and false images are likely to appear because each scintillator element must be used from one end part to the opposite end part in the slice direction.